The present invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube which is capable of preventing the deformation of a color selection electrode such as a shadow mask.
A color cathode ray tube, for example, a shadow-mask type color cathode ray tube employed by a color television set, a color display monitor for an OA apparatus terminal or the like includes an evacuated envelope which is comprised of an approximately rectangular panel portion which has a phosphor screen including a large number of dot-like or stripe-like phosphor picture elements, an approximately tubular neck portion accommodating an electron guns and an approximately funnel-like funnel portion which connects the neck portion and the panel portion. In the evacuated envelope, a shadow mask which constitutes a color selection electrode having a large number of electron beam apertures is disposed. That is, the shadows mask is disposed close to the phosphor screen, faces the phosphor screen in an opposed manner and is. fixedly secured to a mask frame.
This shadow mask mainly uses aluminum killed steel as a constituent material. Recently, along with efforts to seek for higher definition of color cathode ray tubes, shadow masks having a thin plate thickness have been used. With respect to a color cathode ray tube adopting such a shadow mask having a thin thickness, a portion thereof is thermally deformed during its operation and a phenomenon called a mask doming which displaces electron beam spots from given positions on the phosphor screen is liable to occur. To cope with this phenomenon, a shadow mask suspension mechanism has been improved and furthermore Invar material is used as the constituent material of the shadow mask in view of the thermal expansion rate and the physical hardness.
In manufacturing such a shadow mask, an original plate having a large number of above-mentioned electron beam apertures at given positions by etching is blanked out to a given shape. Then, the blanked plate is formed by a press into a shape which includes a main surface having an approximately spherical plane and skirt portions contiguously connected to the periphery of this main surface and are bent at approximately 90 degrees relative to the main surface. Then, this shaped shadow mask is fixedly secured to the above-mentioned mask frame to constitute a mask assembly.
In the above-mentioned shaped shadow mask, a so-called spring back is generated in the skirt portions and hence, a warping is generated on the skirt portions in an outside direction, that is, in a direction to move away from a tube axis. When the skirt portions are fixedly secured to the mask frame portion in such a state, a deformation derived from the warping of the skirt portions is generated at a portion of the main surface of the shadow mask.
Various countermeasures have been proposed in the past to prevent such deformation. That is, first of all, Japanese Laid-open Patent Publication 112566/1974 discloses a technique which prevents deformation by locally thinning the thickness of a peripheral portion of the main surface of the shadow mask. Further, Japanese Laid-open Patent Publication 271849/1988 discloses a technique which prevents deformation by setting the length of the skirt portions of the shadow mask to a specific value relative to the outer diameter dimension of the panel, by providing tongues which protrude from the skirt portions in a direction away from the main surface approximately parallel to the tube axis and by fixedly securing the tongues to the mask frame.
Further, Japanese Laid-open Patent Publication 169847/1989 discloses a technique which prevents deformation by forming a large number of approximately circular small holes in the skirt portions at the comer portions of the shadow mask. Still further, Japanese Laid-open Patent Publication 35657/1997 discloses a technique which prevents deformation by forming a plurality of stress absorption holes in the skirt portions of the shadow mask. Furthermore, a technique which prevents the deformation by thinning the area ranging from the end portion of the main surface to the skirt portions by forming non-through holes or grooves on such an area is disclosed in various publications.
Still further, with respect to the technique disclosed in the above-mentioned Japanese Laid-open Patent Publication 271849/1988 which provides the tongues protruding from the skirt portions in a direction away from the main surface approximately parallel to the tube axis and fixedly secures the tongues to the mask frame to prevent the landing failure caused by the thermal. expansion of the shadow mask, such a technique is also disclosed in Japanese Laid-open Utility Model publication 5657/1973, Japanese Laid-open Patent Publication 73970/1974, Japanese Laid-open Patent Publication 72545/1990, Japanese Laid-open Patent Publication 22048/1992 and the like.
The techniques for reducing the spring back disclosed in the above-mentioned respective publications can expect the deformation prevention effect to some extent compared to structures which do not employ these kinds of techniques.
However, the conventional technique which thins the thickness of the area ranging from the end portion of the main surface to the skirt portions or forms through holes only in the skirt portions has a limit and hence is still insufficient in preventing the deformation. Further, with respect to the shadow mask having the large radius of curvature of the main surface, there has been a problem that the deformation prevention effect can be hardly expected.
Further, with respect to the above-mentioned technique which simply forms tongues protruding from the skirt portions in the direction away from the main surface approximately parallel to the tube axis and fixedly secures these tongues to the mask frame, it gives rise to another problems that the reduction of the spring back of the skirt portions is difficult and the tongues fixedly secured to the mask frame are displaced in a heating step or the like during a manufacturing process of color cathode ray tubes and this causes the deformation of the main surface. The same goes for the techniques employing tongues disclosed in the above-mentioned respective publications and hence, a further countermeasure is requested.
Accordingly, it is a typical object of the present invention to provide a color cathode ray tube which can prevent the deformation of a color selection electrode such as a shadow mask.
To describe the typical constitutions of the present invention for achieving the above-mentioned object, they are as follows.
(1) In a shadow mask (a color selection electrode) including a main surface (apertured region) having a large number of electron beam apertures, skirt portions which are bent perpendicular to this main surface and tongue portions which are protruded from distal ends of the skirt portions in a direction away from the main surface, stress absorption holes are formed in the skirt portions and the tongue portions respectively.
Due to such a constitution (1), a warping amount of the skirt portions of the color selection electrode by the spring back can be reduced and further the displacement caused by a heating step of the tongue portions can be prevented thus eliminating the deflection of the main surface.
(2) The dimensions of the stress absorption holes formed in the skirt portions and the tongue portions are changed.
(3) The dimension of the stress absorption holes formed in the tongue portions is made larger than the dimension of the stress absorption holes formed in the skirt portions.
Due to the above-mentioned constitutions (2)(3), a warping amount of the skirt portions can be reduced and furthermore the problem derived from the above-mentioned displacement of the tongue portions can be absorbed by the stress absorption holes formed in the portions and hence, the deformation of the main surface is suppressed.
(4) The stress absorption holes formed in the skirt portions are formed around the whole periphery except for the corner portions, portions disposed at both sides of the corner portions and the tongue portions of the color selection electrode.
Due to the constitution (4), the height of the skirt portions at the corner portions can be made short and a warping amount of the skirt portions can be reduced.
(5) The stress absorption holes are arranged such that a line which connects the centers of a plurality of stress absorption holes becomes approximately parallel to a line which connects a plurality of fixed points which fix the tongue portions and the mask frame.
Due to the constitution (5), the function to hold the electrodes uniformly along the peripheral direction of the skirt portions can be obtained.
(6) The sum of the opening areas of a plurality of stress absorption holes formed in the tongue portion occupies 25% of the area of the tongue portion.
Due to the constitution (6), the deformation of the main surface derived from the displacement of the tongue portions can be reduced.
(7) The distance from the distal end of the tongue portion to the stress absorption holes is set to equal to or more than 3 mm.
Due to the constitution (7), the occurrence of cracks which may be caused by press forming can be prevented.
(8) Corner pieces provided to the corner portions and the mask frame are fixedly secured to each other.
Due to the constitution (8), fixing of the corner portions of the color selection electrode and the mask frame can be performed together with the tongue portions having the stress absorption holes thus further reducing the deformation of the main surface.
(9) In an approximately rectangular shadow mask (a color selection electrode) including a main surface (apertured region) having a plurality of electron beam apertures, skirt portions which are bent perpendicular to this main surface and tongue portions which are protruded from a distal end of the skirt portions in a direction away from the main surface, the color selection electrode includes stress absorption holes in the tongue portions and equivalent radius of curvature along the long axis of the main surface is set to equal to or more than 1250 mm.
Due to the constitution (9), the distortion of images can be avoided and furthermore the increase of a warping amount of the skirt portions brought about corresponding to the increase of the radius of curvature can be suppressed.
Corner pieces which are fixedly secured to the mask frame are provided to the corners of the color selection electrode.
(10) In an approximately rectangular shadow mask (a color selection electrode) including a main surface (apertured region) having a plurality of electron beam apertures, skirt portions which are bent perpendicular to this main surface and tongue portions which are protruded from a distal end of the skirt portions in a direction away from the main surface, the color selection electrode includes stress absorption holes in the tongue portions and substantially the whole of the color selection electrode is made thin except for a region where the electron beam apertures are formed and portions of the tongue portions.
Due to the constitution (10), the deformation of the main surface is further reduced by the coupled effects consisting of the effect brought about by the stress absorption holes formed in the tongue portions and the thinning effect.